Carbide-derived carbon
Carbide-derived carbon, also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary, or ternary carbides, also known as MAX phases. CDCs have also been derived from polymer-derived ceramics such as Si-O-C or Ti-C, and carbonitrides, such as Si-N-C. CDCs can occur in various structures, ranging from amorphous to crystalline carbon, from sp2- to sp3-bonded, and from highly porous to fully dense. Among others, the following carbon structures have been derived from carbide precursors: micro- and mesoporous carbon, amorphous carbon, carbon nanotubes, onion-like carbon, nanocrystalline diamond, graphene, and graphite. Among carbon materials, microporous CDCs exhibit some of the highest reported specific surface areas. By varying the type of the precursor and the CDC synthesis conditions, microporous and mesoporous structures with controllable average pore size and pore size distributions can be produced. Depending on the precursor and the synthesis conditions, the average pore size control can be applied at sub-Angstrom accuracy. This ability to precisely tune the size and shapes of pores makes CDCs attractive for selective sorption and storage of liquids and gases and the high electric conductivity and electrochemical stability allows these structures to be effectively implemented in electrical energy storage and capacitive water desalinization.
History
The production of SiCl4 by high temperature reaction of chlorine gas with silicon carbide was first patented in 1918 by Otis Hutchins, with the process further optimized for higher yields in 1956. The solid porous carbon product was initially regarded as a waste byproduct until its properties and potential applications were investigated in more detail in 1959 by Walter Mohun. Research was carried out in the 1960-1980s mostly by Russian scientists on the synthesis of CDC via halogen treatment, while hydrothermal treatment was explored as an alternative route to derive CDCs in the 1990s. Most recently, research activities have centered on optimized CDC synthesis and nanoengineered CDC precursors.Nomenclature
Historically, various terms have been used for CDC, such as "mineral carbon" or "nanoporous carbon". Later, a more adequate nomenclature introduced by Yury Gogotsi was adopted that clearly denotes the precursor. For example, CDC derived from silicon carbide has been referred to as SiC-CDC, Si-CDC, or SiCDC. Recently, it was recommended to adhere to a unified precursor-CDC-nomenclature to reflect the chemical composition of the precursor.Synthesis
CDCs have been synthesized using several chemical and physical synthesis methods. Most commonly, dry chlorine treatment is used to selectively etch metal or metalloid atoms from the carbide precursor lattice. The term "chlorine treatment" is to be preferred over chlorination as the chlorinated product, metal chloride, is the discarded byproduct and the carbon itself remains largely unreacted. This method is implemented for commercial production of CDC by Skeleton in Estonia and Carbon-Ukraine. Hydrothermal etching has also been used for synthesis of SiC-CDC which yielded a route for porous carbon films and nanodiamond synthesis.[Image:Figure2CDC2.jpg|thumb|right|Schematic of chlorine etching of to produce a porous carbon structure.]
Chlorine treatment
The most common method for producing porous carbide-derived carbons involves high-temperature etching with halogens, most commonly chlorine gas. The following generic equation describes the reaction of a metal carbide with chlorine gas :Halogen treatment at temperatures between 200 and 1000 °C has been shown to yield mostly disordered porous carbons with a porosity between 50 and ~80 vol% depending on the precursor. Temperatures above 1000 °C result in predominantly graphitic carbon and an observed shrinkage of the material due to graphitization.
[Image:Figure3CDC2.jpg|thumb|right|Different bulk porosity of CDCs derived from different carbide precursors.]
The linear growth rate of the solid carbon product phase suggests a reaction-driven kinetic mechanism, but the kinetics become diffusion-limited for thicker films or larger particles. A high mass transport condition facilitates the removal of the chloride and shifts the reaction equilibrium towards the CDC product. Chlorine treatment has successfully been employed for CDC synthesis from a variety of carbide precursors, including SiC, TiC, B4C, BaC2, CaC2, Cr3C2, Fe3C, Mo2C, Al4C3, Nb2C, SrC2, Ta2C, VC, WC, W2C, ZrC, ternary carbides such as Ti2AlC, Ti3AlC2, and Ti3SiC2, and carbonitrides such as Ti2AlC0.5N0.5.
Most produced CDCs exhibit a prevalence of micropores and mesopores, with specific distributions affected by carbide precursor and synthesis conditions. Hierarchic porosity can be achieved by using polymer-derived ceramics with or without utilizing a templating method. Templating yields an ordered array of mesopores in addition to the disordered network of micropores.
It has been shown that the initial crystal structure of the carbide is the primary factor affecting the CDC porosity, especially for low-temperature chlorine treatment. In general, a larger spacing between carbon atoms in the lattice correlates with an increase in the average pore diameter. As the synthesis temperature increases, the average pore diameter increases, while the pore size distribution becomes broader. The overall shape and size of the carbide precursor, however, is largely maintained and CDC formation is usually referred to as a conformal process.
[Image:Figure4CDC.jpg|thumb|right|Pore size distributions for different carbide precursors.]
Vacuum decomposition
Metal or metalloid atoms from carbides can selectively be extracted at high temperatures under vacuum. The underlying mechanism is incongruent decomposition of carbides, using the high melting point of carbon compared to corresponding carbide metals that melt and eventually evaporate away, leaving the carbon behind.Like halogen treatment, vacuum decomposition is a conformal process. The resulting carbon structures are, as a result of the higher temperatures, more ordered, and carbon nanotubes and graphene can be obtained. In particular, vertically aligned carbon nanotubes films of high tube density have been reported for vacuum decomposition of SiC. The high tube density translates into a high elastic modulus and high buckling resistance which is of particular interest for mechanical and tribological applications.
While carbon nanotube formation occurs when trace oxygen amounts are present, very high vacuum conditions result in the formation of graphene sheets. If the conditions are maintained, graphene transitions into bulk graphite. In particular, by vacuum annealing silicon carbide single crystals at 1200–1500 °C, metal/metalloid atoms are selectively removed and a layer of 1–3 layer graphene is formed, undergoing a conformal transformation of 3 layers of silicon carbide into one monolayer of graphene. Also, graphene formation occurs preferentially on the Si-face of the 6H-SiC crystals, while nanotube growth is favored on the c-face of SiC.
Hydrothermal decomposition
The removal of metal atoms from carbides has been reported at high temperatures and pressures. The following reactions are possible between metal carbides and water:Only the last reaction yields solid carbon. The yield of carbon-containing gases increases with pressure and decreases with temperatures. The ability to produce a usable porous carbon material is dependent on the solubility of the formed metal oxide in supercritical water. Hydrothermal carbon formation has been reported for SiC, TiC, WC, TaC, and NbC. Insolubility of metal oxides, for example TiO2, is a significant complication for certain metal carbides.
Applications
One application of carbide-derived carbons is as active material in electrodes for electric double layer capacitors which have become commonly known as supercapacitors or ultracapacitors. This is motivated by their good electrical conductivity combined with high surface area, large micropore volume, and pore size control that enable to match the porosity metrics of the porous carbon electrode to a certain electrolyte. In particular, when the pore size approaches the size of the ion in the electrolyte, there is a significant increase in the capacitance. The electrically conductive carbon material minimizes resistance losses in supercapacitor devices and enhances charge screening and confinement, maximizing the packing density and subsequent charge storage capacity of microporous CDC electrodes.[Image:Figure5CDC.jpg|thumb|right|Confinement of solvated ions in pores, such as those present in CDCs. As the pore size approaches the size of the solvation shell, the solvent molecules are removed, resulting in larger ionic packing density and increased charge storage capability.]
CDC electrodes have been shown to yield a gravimetric capacitance of up to 190 F/g in aqueous electrolytes and 180 F/g in organic electrolytes. The highest capacitance values are observed for matching ion/pore systems, which allow high-density packing of ions in pores in superionic states. However, small pores, especially when combined with an overall large particle diameter, impose an additional diffusion limitation on the ion mobility during charge/discharge cycling. The prevalence of mesopores in the CDC structure allows for more ions to move past each other during charging and discharging, allowing for faster scan rates and improved rate handling abilities. Conversely, by implementing nanoparticle carbide precursors, shorter pore channels allow for higher electrolyte mobility, resulting in faster charge/discharge rates and higher power densities.